1. Field of the Invention
The present invention relates generally to flexible circuits, and specifically, in an exemplary embodiment, to a flexible circuit having conductive lines, at least one of which having a length extension.
2. Background of the Invention
As an example of one use of a fluid ejection apparatus, the art of printing images with inkjet technology is relatively well known. In general, an image is produced by emitting ink drops from an inkjet cartridge assembly at precise moments such that they impact a print medium at a desired location. In one implementation, the inkjet cartridge assembly is supported by a movable print carriage within a device, such as an inkjet printer, and is caused to reciprocate/scan relative to an advancing print medium and emit ink drops at such times pursuant to commands of a microprocessor or other controller. The timing of the ink drop emissions corresponds to a pattern of pixels of the image being printed. Other than printers, familiar devices incorporating inkjet technology include fax machines, all-in-ones, photo printers, and graphics plotters, and the like. Moreover, technologies pertaining to inkjet have been extended into such diverse fields as printed electronics and micro-fluid medical devices, among other examples of technologies that utilize fluid ejection apparatuses.
Conventionally, an inkjet cartridge assembly includes a housing, a flexible circuit, such as a tape automated bonding (TAB) circuit and a printhead chip (sometimes generically referred to as a printhead). The TAB circuit and the printhead are often attached to the housing. The printhead generally includes ink jetting orifices in operable communication with actuator elements and ink, wherein ink droplets are ejected through the orifices onto the print medium in a known manner. The TAB circuit generally includes a flexible tape-like substrate which supports a plurality of conductive traces. The traces are connected at one end thereof with bond pads of the printhead and at an opposite end thereof with contact pads. By way of example and referring briefly to FIG. 6, a conventional connection/termination between the conductive traces 30 and the contact pads 32 is shown. As shown, a portion of a conventional trace 30 leading to a contact pad 32 is typically relatively straight line and leaves empty/free circuit space surrounding the contact pad 32. The contact pads 32 are often used to engage corresponding electrical terminals on, for example, a movable carriage assembly, such as when an inkjet cartridge is snapped into place, and are used to allow, among other things, actuator elements of the printhead to be actuated to eject the droplets of ink onto the print medium during use. The conductive traces on the TAB circuit are typically in the form of copper traces which are formed via an etching process on a bottom side of the flexible tape adjacent the housing.
A shortcoming associated with conventional TAB circuits is that, in order to produce a high print quality, a balanced electrical current should be passed from the contact pads through the conductive traces and to the bond pads of the printhead (a balanced electrical current should also be passed from the bond pads to any actuated actuator elements as well, but that technology is not part of the present invention). Any imbalance in such a current can result in an imbalance in how the ink is ejected by the inkjet cartridge assembly. This, in turn, may lead to poor image quality.
Conventionally, it might be desired to make the resistance of the conductive traces, specifically the PWR and ground (GRN) traces, of the TAB circuit relatively the same. The conductive traces that extend between the contact pads and the printhead are typically of varying lengths. Therefore, in an attempt to maintain a resistive balance, the conductive traces are typically drawn with widths inversely proportional to their lengths. Typically, the longer traces are wider and the shorter traces are narrower, following that:R=ρL/(w*h)where: R=resistance; ρ=resistivity; L=length; w=width; and h=height.
Unfortunately, there is a limit as to how narrow the shortest traces can be drawn. For example, the dielectric in conventional TAB circuits can only withstand a certain temperature rise caused by the current density passing through the associated conductive traces. Since current density is the current divided by the cross-sectional area of the trace, the width of the traces must be wide enough to accommodate the temperature rise. Known TAB circuits and methods thereof have attempted to draw the longer conductive traces wider, where possible, to compensate for the foregoing disadvantage. Often, due to the physical constraints of some TAB circuits, the longer traces cannot be drawn wide enough to match the resistance of the shortest traces. Therefore, an imbalance in the resistance of the traces might remain. This, in turn, can lead to an imbalance in actuator actuation energy, for example, and may decrease the overall print quality.
In view of the disadvantages of the current methods and apparatus, amongst other reasons, a need still exists for a method and apparatus for providing, for example, a TAB circuit which is constructed such that the conductive traces maintain a resistive balance.